Method and system for initial synchronization and collision avoidance in device to device communications without network coverage

ABSTRACT

A method at a first device for enabling a device-to-device wireless link, the method detecting whether a presence signal of a second device is received over a first time period, the presence signal of the second device having a time-slot boundary; and if the presence signal of the second device is not detected, initiating a time-slot boundary by the first device including: transmitting a first presence signal of the first device in a selected time-slot; and checking for an acknowledgment to the first presence signal.

FIELD OF THE DISCLOSURE

The present disclosure relates to wireless device-to-device (D2D)communications, and in particular relates to device-to-devicecommunications without a controlling network infrastructure node.

BACKGROUND

In current wireless network scenarios, a device will typicallycommunicate with a network infrastructure node such as a base station oran access point that the device is being served by, which will thenallow communication to other devices, including those served by thatsame network infrastructure node, or to devices served by other networkinfrastructure nodes. However, such communication may not be possible incertain areas where wireless network coverage from an infrastructurenode does not exist, for example, remote areas without wireless networkdeployment or areas which have suffered a destruction of wirelessnetwork infrastructure. Further, even where wireless network coverageexists, communications using a network infrastructure node may not bedesirable. For example, in general communication systems, a direct D2Dtransmission of data may provide more efficient utilization of radioresources than current networks.

Device-to-device communications are communications between two wirelessdevices or user equipments (UEs), where the communication proceedsdirectly between UEs and does not proceed through a networkinfrastructure node. Uses for D2D communications may be for bothemergency and non-emergency situations. For example, first respondersand public safety members may use D2D communications to communicatebetween devices. This may be useful in situations where there is nonetwork coverage, such as remote areas or inside a building. However,even in network coverage areas, in some cases D2D communications aredesirable in public safety situations.

In non-emergency situations, friends that are in close proximity to eachother may wish to communicate directly with each other. Other casesinclude human-to-machine interaction, such as parking meters talking tomobile wireless devices within range to help a user of a mobile wirelessdevice find a free parking space. Machine-to-machine communication ispossible as well, for example, temperature/humidity/pressure sensorcommunicates recorded data to a controller device. Other examples arepossible. The devices involved may be stationary or mobile.

The operation of a device for communicating with other devices without anetwork infrastructure element however has challenges, since there is nocentral control for such communications.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood with reference to thedrawings, in which:

FIG. 1 is a block diagram illustrating the provision of a presencesignal within a time-slot;

FIG. 2 is a block diagram showing a time-slot in which a first radioframe is used to transmit a presence signal and a second radio frame isused to transmit an acknowledgement;

FIG. 3 is a block diagram showing a single carrier is used fortransmitting presence signals and acknowledgements for a discoveryperiod;

FIG. 4 is a block diagram showing two carriers, a first used fortransmitting presence signals and a second used for transmittingacknowledgements;

FIG. 5 is a block diagram showing radio frames for transmission ofprimary synchronization signals, second synchronization signals, andacknowledgements;

FIG. 6 is a block diagram showing a plurality of radio frames fordifferent devices;

FIG. 7 is a block diagram showing an SC-FDMA radio frame fortransmitting a primary synchronization signal, a secondarysynchronization signal, and acknowledgements;

FIG. 8 is a process diagram showing a device process for implementingone embodiment of the present disclosure;

FIG. 9 is a block diagram showing the setting of a time-slot boundary;

FIG. 10 is a block diagram showing the selection of an unused time-slot;

FIG. 11 is a plot of the convergence of a plurality of networks havingdifferent numbers of devices and different numbers of maximum devices;

FIG. 12 is a block diagram showing time-slot boundary misalignment;

FIG. 13 is a block diagram showing a stochastic hidden node problem;

FIG. 14 is a process diagram showing a process for an embodiment wherelink failures are possible;

FIG. 15 is a process diagram showing functionality for an exampleboundary establishment block;

FIG. 16 is a process diagram showing functionality for an example PStransmission block;

FIG. 17 is a process diagram showing functionality for an exampleacknowledgement transmission block;

FIG. 18 is a plot of the convergence of a plurality of networks havingdifferent numbers of devices and different numbers of maximum devices;and

FIG. 19 is a block diagram of an example user equipment which may beused with the embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure provides a method at a first device for enablinga device-to-device wireless link, the method comprising: detectingwhether a presence signal of a second device is received over a firsttime period, the presence signal of the second device having a time-slotboundary; and if the presence signal of the second device is notdetected, initiating a time-slot boundary by the first device including:transmitting a first presence signal of the first device in a selectedtime-slot; and checking for an acknowledgment to the first presencesignal.

The present disclosure further provides a device for enabling adevice-to-device link, the device comprising: a processor, wherein theprocessor is configured to: detect whether a presence signal of a seconddevice is received over a first time period, the presence signal of thesecond device having a time-slot boundary; and if the presence signal ofthe second device is not detected, initiate a time-slot boundary by thefirst device including: transmitting a first presence signal of thefirst device in a selected time-slot; and checking for an acknowledgmentto the first presence signal.

The present disclosure further provides a method at a first device forenabling a device-to-device wireless link, the method comprising:listening for a presence signal on a channel, the presence signalcomprising at least one sequence; and upon detection of the presencesignal, transmitting an acknowledgement to the presence signal; andaligning to a time-slot boundary associated with the presence signal byutilizing at least one sequence of the presence signal.

The present disclosure further provides a device for enabling adevice-to-device wireless link, the device comprising: a processor,wherein the processor is configured to: listen for a presence signal ona channel, the presence signal comprising at least one sequence; andupon detection of the presence signal, transmit an acknowledgement tothe presence signal; and align to a time-slot boundary associated withthe presence signal by utilizing at least one sequence of the presencesignal.

The present disclosure further provides a method at a device forenabling a device-to-device wireless link, the method comprising:listening for a presence signal from another device on a channel; andtransmitting a presence signal on the channel to establish a time-slotboundary, wherein the transmitting of the presence signal enablesanother device to detect such presence signal and to align to theestablished time-slot boundary.

The present disclosure further provides a device for enabling adevice-to-device wireless link, the device comprising: a processor,wherein the processor is configured to: listen for a presence signalfrom another device on a channel; and transmit a presence signal on thechannel to establish a time-slot boundary, wherein the transmitting ofthe presence signal enables another device to detect such presencesignal and to align to the established time-slot boundary.

Device to device applications and services, also referred to herein asproximity-based applications and services, represent an emerging socialand technological trend. In this regard, the 3^(rd) GenerationPartnership Project (3GPP) Long Term Evolution (LTE) architecture isevolving to include such services, which would allow the 3GPP industryto serve the developing market and, at the same time, serve urgent needsof various Public Safety communities.

However, the implementation of a network between devices without acentral controller has various issues. One issue with regard to deviceto device communications is the discovery of devices outside networkcoverage and the ability for devices to communicate with each other. Inparticular, currently there is no way to achieve reliable discovery ofdevices in an ad hoc network having no controlling network element. Suchdiscovery allows devices in the network to be continually aware of thepresence of other devices with which they can communicate directly.Further, for transmission of a signal, presently there is no clear wayto establish a common time frame synchronization for long term evolutionorthogonal frequency division multiplexing (OFDM) or single carrierfrequency division multiple access (SC-FDMA) symbol transmission orreception in the absence of any network infrastructure.

A further issue for D2D communications relates to fixed lengthtransmissions, where time-slots are used. Periodic discovery signalstransmitted by devices may take up less than one time-slot (TS), andcurrently it is not clear how to establish the time-slot boundaries andradio frame timing to align the transmission of numerous devices in anad-hoc network.

As used herein, a time-slot is a generalized term of a fixed length timewindow that may be used by a device to communicate with other devices,and differs from a “slot” as used by LTE specifications.

A further issue is that currently devices do not detect and resolvepotential uplink transmission conflict for device to devicecommunications when two or more devices both select the same initialtime-slot for transmission. Therefore, a collision resolutionmethodology is provided herein.

The present disclosure therefore provides for the initialization of awireless network for device to device communication outside of networkcoverage. Specifically, the present disclosure addresses how a group ofdevices can discover each other, define time-slot boundaries, assigntransmission time-slots to each device, avoid and resolve collisions,and synchronize with each other, among other features. Theseinitialization steps occur before devices are able to have datacommunication between each other. Further, the embodiments describedherein apply to either fully connected networks or partially connectednetworks, such as where not all devices are able to receivetransmissions from all other devices in the network.

While the present disclosure provides examples utilizing the LTEarchitecture, the embodiments described herein are not limited to sucharchitecture, and other network architectures could equally be used. Thetechniques and examples provided herein could therefore be expanded toother technologies besides 3GPP long term evolution (LTE).

When a system is in a discovery stage, devices are not connected to anetwork, and thus no synchronization may have been established. Inaccordance with one embodiment of the present disclosure, to addressthis, devices may periodically transmit presence signals (PS) with fixedlength that can be transmitted within time-slots. The configuration ofsuch presence signals is provided below.

Upon receiving an identifiable PS, an acknowledgement signal (ACK) maybe sent back to the device transmitting the PS. The ACK, as describedbelow, is designed so that multiple receivers can respond to the PS bytransmitting the same ACK. Further, in accordance with the presentdisclosure an algorithm is provided to align devices to the sametime-slot boundary. Note that while the description here focuses onacknowledgement (ACK) as an example, the same transmission can often bedesigned to transmit negative acknowledgement (NACK) as well. Forinstance, the transmission can use two possible sequences (e.g.,all-zeros sequence for ACK vs all-ones sequence for NACK). While NACKcan be utilized to support PS transmission as well, for simplicity thediscussion primarily focuses on ACK. It is intended that usage of NACKin network establishment is covered by the procedures and protocolsdescribed herein.

In accordance with a further embodiment, to obtain time frequencysynchronization for LTE OFDM/SC-FDM symbol transmission/reception, thepresent disclosure provides the use of an existing primarysynchronization signal (PSS) as used in LTE to detect symbol boundarieswithin a time-slot.

In accordance with a further embodiment, TS level synchronization may beachieved through a procedure that allows the first transmit device toestablish a TS boundary reference for other devices in the network. Bydoing this, a slotted time division duplex (TDD) system is established.

In accordance with a further embodiment, in order to detect thetransmission of multiple PS signals on a same time-slot, a randomhopping secondary synchronization signal (SSS) scheme is used to assigna new SSS to a presence signal in every cycle. Here cycle refers to atime frame used by relevant devices as time references to transmit andreceive signals. For example, certain signals (e.g., a broadcast signal)may repeat regularly every cycle, or every integer number of cycles. Inthis application, cycle is also referred to as a frame. A receivingdevice can correlate to the possible SSSs one by one and detect themultiple transmissions of PS signals.

In accordance with a further embodiment, a protocol is provided for UEsto periodically transmit presence signals and to get acknowledgmentsfrom neighbors. For example, a transmitting device will continue toutilize a time-slot that it is currently using if it receives at leastone ACK, and will jump to a new free time-slot if it does not receive anACK for a certain duration. Once the system converges to a stable state,all UEs can periodically broadcast their PS free of collisions. Thus theabove algorithm may be totally distributed.

As used herein, a mobile device, device, user equipment, or other suchterm is interchangeable and refers to a device that is capable ofestablishing device-to-device communications.

Reference is now made to FIG. 1. In order to allow devices in thenetwork to be continually aware of the presence of other devices withwhich they can communicate, in accordance with one embodiment of thepresent disclosure, each device transmits a substantially periodicpresence signal. Though the presence signal may collide with thepresence signals of other devices when a device first startstransmitting, after a contention resolution procedure as describedbelow, the PS transmitted by each device is unique in a time-slot. Inthis way, each device is individually identifiable by its peer devicesfor as long as the device is active and each device is able to obtain adedicated time-frequency resource without continuous collisionresolution.

A discovery signal, in accordance with the present disclosure, involvesone of two signals. A first signal that forms a discovery signal is thepresence signal, which is sent out by a device. A device broadcasts itsPS to notify its presence and further to reserve a time-slot. A seconddiscovery signal is an acknowledgement signal (ACK) received by thedevice. All or a subset of devices that decode the PS broadcast an ACKto indicate approval of the time-slot.

Thus, as illustrated in FIG. 1, a particular time-slot 110 may haveeither one unit per time-slot, as shown by reference 120 or two unitsper time-slot as shown by reference 130. In either of the embodiments ofFIG. 1, each unit contains a synchronization signal 140, a discoverysignal 142, which may comprise either a presence signal or anacknowledgement, and a data payload 144.

A plurality of time-slots are combined to form a discovery period 150and the discovery period time for the periodicity of the discoverysignals is defined to be the number of time-slots per discovery period(N_(Max)) times the time (Ts) per time-slot. The discovery period canhave the same length as a frame. The frame is used as time reference forboth initial discovery and subsequent data communication.

Thus, in accordance with the present disclosure, the discovery signalsare repeated on each discovery period. Each discovery period is composedof N_(Max) time-slots, where a device can either transmit or receive aradio-frame on any given time-slot. The N_(Max) may represent themaximum number of UEs that can be discovered in accordance with thedesigned architecture.

Before devices discover each other, the data payload time unit 144remains empty.

In accordance with the present disclosure, two system designs aredescribed. However, such systems are merely examples and other systemscould also be used. In a first system, a one carrier frequency timedivision duplex system is provided, where two units per time-slot areused. In a second system, a two carrier frequency time division duplexsystem is used where there is only one unit per time-slot.

One Carrier Frequency TDD System

In accordance with one embodiment, a UE operates in a D2D mode usingonly one carrier frequency (e.g., either an uplink channel or downlinkchannel, as for example defined in LTE) for both transmission andreception. As used herein, uplink is defined similarly to a cellularsystem where uplink refers to transmissions from a device. Similarly,downlink involves signals received at the device.

Reference is now made to FIG. 2, which shows an embodiment of a channelin accordance with one embodiment of the present disclosure in whicheach time-slot is divided into a transmission portion and a receptionportion from the point of view of a given device. Typically otherportions of the time-slot could be used for other purposes such as dataor synchronization signals, and thus the embodiment of FIG. 2 is merelya simplification.

As seen in FIG. 2, three time-slots are provided where NMAX from aboveis defined as three. Thus, the system of FIG. 2 has converged to asolution with three devices in which the time-slots repeat themselves.

In accordance with FIG. 2, each time-slot has two transmission instanceswhere the first transmission instance is for the PS signal and thesecond transmission instance is for the ACK signal. In accordance withone embodiment, each time-slot may be 2 ms long. Based on LTEspecifications, a system is provided such that a UE may transmits its PSand should receive an ACK a certain number of milliseconds later. Forexample, in accordance with the embodiment of FIG. 2, the ACK isreceived 3 ms after the transmitting of the signal.

In particular, in a first time-slot 210 a PS signal is sent by a mobiledevice A shown by reference 212. 3 ms later, as shown by reference 214,device B and/or C sends an acknowledgement if they received the PSsignal correctly.

In time-slots 220 a device B transmits its PS signal in the firsttransmission instance, as shown by reference 222, and 3 ms later, asshown by transmission instance 224, devices C and/or A send an ACK ifthe PS signal is received correctly.

Similarly, in time slots 230, in a transmission instance 232 device Ctransmits its PS signal and in transmission instance 234, devices Aand/or B send their ACK.

The discovery period in this case is 6 ms, where three time-slots areprovided with each time-slot having 2 ms.

Variations of the above are possible. Further, the timing could changeand the examples above are provided merely for illustrative purposes.

Several variations of the embodiment of FIG. 2 may include thevariability of the periodicity of the discovery signals including the PSand ACK for a given device. In one embodiment, the periodicity of thediscovery signal may be short (N_(Max)) when a device is attempting aninitial connection. However, once established, the discovery signalperiodicity may be switched to a long period (for example 4×N_(Max)) toreduce the number of transmissions or receptions of the discoverysignal. The reduction in the number of transmissions or receptionshowever means that devices that are trying to connect to the networkwill need to monitor the channel for a longer period to ensure that thetransmission instance that device selects is not occupied by anotherdevice.

In a further variation, not all peer devices need to respond to adevice's PS transmission. For example, in some embodiments only peerdevices related to the transmitting device in some way may need to sendan ACK in response to the transmitting device's PS. This may beparticularly useful, for example, when device-to-device connections arestably established. For example, the response may be based on a deviceidentifier once devices have been assigned an identifier in an ad hocnetwork where devices with certain identifiers need to respond incertain subframes. In another embodiment, a timing based relationshipmay be utilized such that devices with certain PS transmittingopportunity coming up may need to send ACKs. Other examples arepossible.

Similarly, referring to FIG. 3, a system is shown in which N_(Max)=6 andthe acknowledgement occurs 5 ms after the transmitting of the PS. Inparticular, as seen in FIG. 3, in transmission instance 310 a firstdevice transmits a PS and this is acknowledged in radio frame 312.Similarly, in transmission instance 314 a second device transmits a PSand this is acknowledged in transmission instance 316. Othertransmission instances are similarly used for transmission andacknowledgement.

After a discovery period of 12 ms in the example of FIG. 3, the processrepeats itself.

The above embodiments of FIGS. 2 and 3 assume that each time-slot isfull. In other embodiments, some time-slots may be unoccupied, e.g., toallow for new devices to be added.

Two Carrier Frequencies TDD System

In an alternative embodiment to that described with reference to FIGS. 2and 3 above, a UE may have both uplink and downlink channels, where eachof the channels is capable of both transmission and reception. In thiscase, for D2D communications one carrier may be allocated for PS signalsand the other channel/carrier may be allocated for ACK signals.Reference is now made to FIG. 4.

In the example of FIG. 4, a time slot is defined as 1 ms and 12time-slots are shown for each of the PS and ACK channels. In a time-slot410, a first device transmits its PS and this is acknowledged 3 ms lateras shown by reference 412.

Similarly, in the second time-slot a second device transmits its PSsignal, as shown by reference numeral 420 which is then acknowledged intime slot 422.

Time-slots can be occupied or unoccupied and the acknowledgement mayoccur 3 ms later in the corresponding time-slots on the acknowledgementchannel.

The timing relationship between the PS and the ACK may vary depending onthe implementation. Thus, a PS is not necessarily responded to in theACK in the closest time. A PS is typically acknowledged by an ACK signalseveral time-slots away in order to allow for propagation time andtransmission/reception processing times. The exact timing relationshipbetween a PS and an ACK may adopt various values, such as a 4 msseparation used in LTE frequency division duplex (FDD) systems in oneembodiment. Other values are also possible.

At any given time-slot a UE can either be in transmitting mode orreceiving mode. In a transmitting mode a UE either transmits a PS signalor an ACK signal. Similarly, in a receiving mode a UE detects the PSsignals, the ACK signals and collisions of PS signals or an emptytime-slot. Collisions may occur when two or more UEs transmit a PS inone time-slot. Moreover, due to the fact that a UE cannot transmit andreceive at the same time (full-duplex UE is not assumed here), a UE in atransmitting mode cannot detect a collision with other UEs. However,those UEs that are in the receiving mode could detect such collision. Inone embodiment, the UEs in the receiving mode will not send anynotification, but the transmitting UE may use a confidence measure asdescribed below to detect the collision. In other embodiments, the UEsin the receiving mode may duly notify the transmitter UEs.

By partitioning the radio resource into PS time-slots and ACK time-slotsas described with reference to FIG. 4, a feedback mechanism isaccommodated in accordance with the present disclosure. In particular,if the UE in the receiving mode detects a collision, then it does notsend back an ACK signal in one embodiment. However, if there is nocollision, the receiver UE may broadcast an acknowledgement signal on anACK time-slot after a TACK duration of time. The transmitter UE mustwait to receive the ACK signal before securing its time-slot. If thetransmitter UE does not receive an ACK, in ideal conditions it mayassume that a collision has occurred and thus find a new time-slot toretransmit the PS. In non-ideal conditions, if an ACK is not received,confidence by the transmitting UE in the time-slot may decrease until itfalls below a threshold, at which point the transmitting UE will lookfor a new time-slot to retransmit the PS.

Frame Structure And Signal Design

In accordance with one embodiment of the present disclosure, the primarysynchronization signal (PSS) may be used for dual purpose of both apreamble for a time/frequency synchronization and also as a primarypresence signal. The secondary synchronization signal (SSS) may be usedas secondary presence signal and as a temporary device identifier.

According to current LTE standards, three possibilities exist for PSS.For device-to-device communication the PSS value can be utilized toidentify a device-to-device cluster. In particular, a cluster is a groupof devices that are able to establish direct links amongst them. In oneexample, each device in the group is a one-hop neighbor of all otherdevices in the group in accordance with the present disclosure.

As there are 168 possible values of an SSS in LTE, the SSS may beutilized as a type of temporary device identifier to enable enhancedcollision detection techniques as described below.

On each discovery cycle, a device may transmit a randomly picked SSScode word. During the discovery phase, if a collision occurs betweenpresence signals of two devices, then other devices are capable ofdetermining that a collision has occurred. Further, the other devicesmay be able to tell how many SSS signals have collided.

While the above describes 3 and 168 sequences for PSS and SSS,respectively, other numbers of sequences may be possible, depending onthe implementation. Instead of having 168 sequences for identification,only 40 may be needed as a given number of devices simultaneously activefor a cluster may not be expected to exceed 40, for example. Using thesmaller sequence space reduces the processing burden on receivingdevices.

From the point of view of a single device, the frame structure is shownwith regard to FIG. 5 below, where a first device is used as an example.In the example of FIG. 5 a first device transmits on a first subframe.Here the subframe as defined in LTE is used as an example, which isanalogous to a time-slot in general. The subframe is shown enlarged atreference numeral 510.

In the first subframe, the first 3 symbols are used for a control region512. Further, since the device is transmitting in that subframe, the PSS514 as well as a randomly chosen SSS 516 may be transmitted in thesubframe.

In the next subframe, if an ACK is required then the ACK may sent asshown by reference numeral 520. Note that while in the figures, ACK isshown as transmitted in the subframe right after the subframe with PStransmission, it is understood that other time delays for ACKtransmission are possible, e.g., ACK is sent 4 subframes after thesubframe with PS transmission.

Thus, in accordance with FIG. 5, the presence signal of a given deviceis always transmitted in an even-numbered subframe, while the ACKresponse is transmitted in an odd-numbered subframe. However, othertiming relationships are possible as defined above.

From the point of view of an entire cluster, the frame structure isshown with regard to FIG. 6, where the presence signal and thecorresponding ACK of three devices are illustrated. In particular, asseen as in FIG. 6, the subframe 610 is used for a first device totransmit, subframe 612 is used for a second device to transmit, andsubframe 614 is used for third device to transmit.

The primary presence signal of FIGS. 5 and 6 can reuse the constructionof an existing LTE sequence d(n) defined for the primary synchronizationsignal. To avoid confusion with PSS sent by base stations, differentvalues may be chosen for the Zadoff-Chu root sequence index u when usedfor device-to-device communication. One example is to choose alternativeindices values as a set of different u values for device-to-devicecommunication. For example, indices values of 40, 41, 23 may be used inone embodiment.

The reuse of existing PSS structures for the purpose of D2Dcommunication further has the benefit of minimizing deviceimplementation since devices are adapted to decode such existing PSSsignals and thus receiving circuitry on the device may stay the same.

For secondary presence signals, these signals may reuse the constructionof an existing LTE sequence d(n) defined for the secondarysynchronization signal. To avoid confusion with the SSS of basestations, the value of the indices (m₀, m₁) can be chosen differentlyfor the device-to-device communication.

The ACK/NACK can be transmitted reusing existing LTE physical hybridacknowledgment repeat request (HARQ) indicator channel (PHICH)construction. After repetition coding, a block of binary phase shiftkeying (BPSK) modulation symbols z(0), . . . , z(M_(s)−1) may besymbol-wise multiplied with an orthogonal sequence and scrambled,resulting in a sequence of modulation symbols d(0), . . . ,d(M_(symb)−1) according to

d(i)=w(i mod N _(SF) ^(PHICH))·(1−2c(i))·Z(└i/N _(SF) ^(PHICH)┘)  (1)

The parameters M_(s), M_(symb), N_(SF) ^(PHICH) are reused from the LTEdefinition. The cell-specific scrambling sequence c(i) is fixed for thepurpose to responding to PS. The orthogonal sequence w(i) is also fixed,e.g., [+1 +1 +1 +1]. Further, due to a design feature of PHICH, multipleACK/NACKs can be multiplexed together. This can be useful whenevermultiple ACK/NACK (or multiple ACKs) are to be sent. For example, thismay occur if: the PS of two devices (separate either in time orfrequency) are responded to in a same TS to save resources for sendingACK; or when there is a need to send ACK/NACK to both PS and datapackets, when PS and data packets coexist after the initial networkestablishment is done. In this case, two or more sets of ACK/NACK (orACK alone) can be combined for transmission, with each set using adifferent orthogonal sequence w(i), e.g., [+1 +1 +1 +1] and [+1 −1 +1−1]. A rule can be specified to predefine the mapping between atransmission (PS or data packet) and a PHICH transmission.

In a further embodiment, devices may use SC-FDM instead of OFDM tomodulate the signal. In this case, the frame structure is shown from thepoint of view of a first device in FIG. 7. As seen in FIG. 7, the PSS712 and SSS 714 are transmitted during a subframe 710, and theacknowledgement 720 may be provided in a control region 730.

Thus, the same PSS, SSS design indicated above for OFDM can also be usedin SC-FDM transmission without change. For the ACK/NACK, the physicaluplink control channel (PUCCH) type of transmission may be used instead.

Device Process

The above may be implemented in accordance with the embodiment of FIG.8. In particular, the process of FIG. 8 starts at block 810 and proceedsto block 812 in which a check is made to see whether a TS has yet beenestablished for the device. If no, the process proceeds to block 814 inwhich a random contention time τ is picked and the process then proceedsto block 816 in which the device listens for a time period designated asD+τ where D is the discovery period (frame period) length. Here ‘listen’refers to the function where a device performs detection of a potentialsignal, to see if such a signal has been transmitted. Similarly, thismay be referred to as a device ‘checks’ for the potential signal.

In particular, at the initial stage when no TS has yet been established,all devices initiate to form a cluster. The main goal at this stage isto establish a TS boundary so that devices can find a TS to transmit thePS. In accordance with the embodiment of FIG. 8, a first collision freePS establishes the TS boundary, which then becomes a reference boundaryfor all devices to line up their TSs.

This is shown with reference to FIG. 9. In particular, as seen in FIG.9, a device starts at a device chosen time, labeled T0, and listens fora discovery time D, which corresponds with the discovery period (frameperiod), plus the randomly picked contention time τ. In this case, τ ischosen to be uniformly distributed between 0 and D.

As seen in FIG. 9, at the time T0+D+τ, labeled T0′, the device has notdetected a signal from another device, and the device thereforetransmits a PS to initiate the TS boundary, as shown with reference 910.

The device then listens for an acknowledgement. If an acknowledgement isreceived to PS transmission 910, then the device establishes the TS slotboundary and continues along timeline 920 to periodically transmit itssignal at a time T0′+XD, where X is a positive integer, as shown byreference numerals 922 and 924.

Conversely, if no acknowledgement is received to the PS transmitted atreference 910, then the device follows timeline 930.

In the embodiment of FIG. 9, timeline 930 shows the embodiment where thedevice starts a new probing period of length D+τ where the contentiontime τ is a newly picked randomly value between 0 and D. In this case,the device detects a PS signal 932 transmitted by another device andaligns its TSs to the TS slot boundary established by another device.The device abandons its listening for the remainder of its probingperiod, and switches to the mode of selecting one of the free TSs totransmit its PS. After T_(Ack) duration the device sends acknowledgementto PS signal 932.

Referring again to FIG. 8, the above is shown with regard to blocks 814and 816, in which the device listens for a time D+τ.

The device then checks at block 818 whether or not a presence signal hasbeen received during the listening time period. The check at block 818may need to be performed continuously during the entire listening time.

From block 818, if a PS is received, then the process proceeds to block820 and a TS boundary is established.

Conversely, from block 818 if no PS is received, the process proceeds toblock 822 in which the device itself transmits its PS to try toestablish TS boundaries. The process then proceeds to block 830 in whicha check is made to determine whether the acknowledgement time has yetoccurred. If not the process proceeds to block 832 in which the devicestays idle until, at block 830, it is found that the acknowledgementtime has arrived. At this point, the process proceeds to block 834 inwhich a check is made to determine whether an acknowledgement has beenreceived. If an acknowledgement has been received, then the processproceeds to block 836 in which a TS boundary is established.

The broadcast of the PS at block 822 or with regard to message 910 ofFIG. 9, may include a system frame number (SFN) in addition to, orconcurrently with a PSS and/or SSS. The SFN may be broadcast to trackthe number of discovery periods (frame) that had elapsed where the SFNmay be set to 0 for a first transmission.

After establishing the TS boundary, the device that established the TSboundary may periodically broadcast the PS and the SFN on a first TS ofevery discovery period. The SFN may be incremented by 1 for each frame.

However, if the device does not receive the ACK in response, it assumesthere has been a partial or total collision of PS transmission withother devices, or there was no other device around. Thus, the TSboundary is not established and the device has to start probing thechannel all over again to see if some other device has successfullyestablished the TS boundary.

The random contention time τ may be different for each TS boundarycreation attempt to avoid collisions with other devices.

In some embodiments, if the device cannot establish or find a TSboundary after K attempts, the device aborts the transmission as itconcludes that there is no other device in the coverage area toestablish a D2D network.

Once the TS boundaries are established, the device can then look for aTS to transmit in. Thus, referring again to FIG. 8, if the PS isreceived and a TS boundary exists as shown by block 820, the device thenproceeds to block 840 and listens for one discovery cycle to determine anew TS to transmit its PS. Thus, the device can listen and determinewhether it sees any empty TSs (i.e., free TS). At block 842 a check ismade to determine if any free TSs exists and if not, the process ends atblock 844 since the cluster is full.

Conversely, at block 842 if there is a free TS detected then the devicemay randomly pick a free TS as shown with block 846. From block 834 ifno ACK is received, or from block 836 or block 846 the process proceedsto block 850 in which the device moves to the next TS for an action.

From block 850, the process proceeds to block 852. Similarly, if a TShas been established previously as shown in the check of block 812 thenthe process may proceed directly to block 852. At block 852, a check ismade to determine whether the current TS is a discovery TS for thedevice. In particular, the check at block 852 determines whether the TSis used for the device to transmit the PS or to receive an ACK. If yes,the process proceeds to block 854 in which a check is made to determinewhether the TS is the time-slot used to transmit the PS.

From block 854, if the TS is for transmitting a PS, the process proceedsto block 856 in which the PS is transmitted and the process thenproceeds to block 850 in which the device goes to the next TS.

Conversely, from block 854, if the TS is not the time-slot used totransmit the PS, then a check is made to determine whether an ACK wasreceived at block 860. If an ACK is received then the process proceedsto block 850 in which the process goes to the next TS. Otherwise, if noACK is received then the device may have had a collision and the processproceeds to block 862 in which the device listens for one cycle to picka new TS. The process then proceeds to block 842 in which a check ismade to determine if there are any free TSs and if no the process endsat block 844. If there are free TSs, then the device randomly picks afree TS at block 846 and then proceeds back to block 850 in which theprocess proceeds to the next TS.

From block 852, if the TS is not the one designated for the device totransmit the PS or to receive an ACK then the process proceeds to block870 in which a check is made to determine whether a PS has been receivedin that TS. If yes, a check is made to determine whether a collisionexists as shown by block 872. If a collision exists then the processgoes to the next TS at block 850.

If no collision is found at block 872 then the process proceeds to block874 in which an ACK is transmitted. As will be appreciated, the ACKtransmission at block 874 may be done in several subframes in the futurein order to ensure ACK timing as is provided above.

Thus, in accordance with the above, when a device detects a presencesignal in a probing period, the device recognizes that the TS boundaryhas been established and the device then needs to listen to the channelto detect pre-existing presence signals. The probing period could be oneor more integer multiples of the discovery cycles period, if each devicetransmits in each discovery cycle, then only one cycle needs to belistened. However, if as indicated above, a device transmits once everymultiple cycle then that multiple needs to be listened to in order toensure the free TS.

The listening for the TS boundary and for TSs allows the device to alignwith the time-slots partitioning in each discovery period and derive alloccupied and available time-slots. The number of time-slots that areoccupied means that there are that number of UEs already established insuch discovery period.

The selection of a time-slot to attempt a PS transmission may be randomwithin available TSs to avoid having two devices that are looking tojoin the network to collide by choosing a specific time-slot. In otherwords, of the remaining time-slots available, the device randomly picksone of the time-slots that is available to enhance the chances ofavoiding a collision.

The process of FIG. 8, and in particular the action at block 874, allowsthe device to either transmit an ACK or transmit nothing. Thus, thereare two states, ACK and discontinuous transmission (DTX).

In an alternative embodiment, three possible responses are available. Inthis case, an ACK, NACK or DTX may be used. However in such case it maybe difficult since a device does not know whether or not another devicewill transmit in a particular time-slot or not and thus, if no PS signalis detected, the receiving device may have no way to tell whether eithernobody transmitted a PS or the transmitted PS was not received properly,for example due to channel conditions, or two devices transmit a PS atthe same time and collided, thus creating interference. In this case,not sending a NACK may improve system performance.

Reference is now made to FIG. 10 which shows the selection of the TS inaccordance with the process of FIG. 8.

In particular, a device has a probing period 1010 of length D in whichit detects PS signals 1012 and 1014.

At the end of probing period 1010 the device randomly picks a free TSand transmits in the selected TS, as shown by reference 1020. The devicethen listens for ACKs after a predetermined duration T_(Ack) and if anACK is received then the device proceeds along the time line 1030 inwhich the same TS is then selected for the device and the devicecontinues to transmit in that TS for each discovery period.

If no ACK is received after the T_(Ack) duration for the correspondingPS transmitted at reference 1020, the device proceeds along timeline1040. On this timeline, a new probing period 1042 of length D is startedand at the end of the probing period a free TS is randomly selected. Inthis case, a transmission 1044 may be made and an ACK listened for. Ifan ACK is received after the T_(Ack) duration, then the device securesthe TS and continues to transmit its PS in the TS during each subsequentdiscovery period.

Referring to FIG. 11, simulation of the above in order to determine howlong a system takes to converge to a stable state wherein a UE reservesits own time-slot for discovery is shown with regard to the number ofdevices and the average number of cycles. In the simulation, it isassumed that all UEs have a direct link to other UEs in the network,thus creating a fully connected cluster. The simulation abstracts forpath loss and decoding errors into a link failure rate, and failure isassumed to be 0. Other simulations where the link failure is not 0 aredescribed below.

Four scenarios were simulated, namely N_(max) being 10, 20, 50 and 100.For each value of N_(max), the network was simulated with variousnumbers of devices from 2 to N_(max). Multiple runs were made for eachvalue and the average was taken for convergence. In the simulation, allUEs were initially unsynchronized. The network convergence means thatall devices have aligned to the same TS boundary and that each devicehas secured a contention-free time-slot to transmit its PS.

As seen in FIG. 11, the increase in the number of devices leads to anincrease in the number of cycles required to reach convergence. Howeverthe increase is linear when the ratio of the number of devices to thetotal number of time-slots is low. As the ratio of the total number ofdevices to the total number of time-slots approaches 1 then the increaseis more exponential since the likelihood of PS collision between devicesincreases exponentially as well. Also, it can be observed that for afixed ratio of the total number of devices to the total number oftime-slots, the network convergence time is longer for the network withmore devices. This means that larger networks require longer times toconverge.

Network With Link Failures

The above embodiments assumed that the network is operating under idealconditions and with the assumption that the network was fully-connected.In other words, the above assumed a direct error-free link from onedevice to any other device in the network.

In accordance with one embodiment of the present disclosure, the aboveassumptions are relaxed by allowing links to have a certain failurerate. This means that when a PS signal is transmitted, there is apossibility that it may not be decodable by some devices in the network.

The removal of the assumption of a direct, error free link leads to twomain issues. A first is multiple TS boundaries (multiple leader devices)in the network, and a second is a stochastic hidden node problem. Inaccordance with the present embodiment, the process from FIG. 8 ismodified to accommodate these issues.

Further, if a re-designed process is robust to link failure on ACKlinks, it may also be robust enough to an occurrence of an ACK totalcancellation due to signal superposition.

The first issue in accordance with the above is multiple TS boundariesdue to link failures. This issue arises when a TS boundary is beingestablished. When a device transmitting is the first one whose presencesignal is used as a reference to establish the TS boundary, the devicethat transmits such signal is called the leading device or the leader.As link failures may occur during that first transmission, some devicesmay not receive the PS signal. As a result, one of those devices maytransmit its own PS signal to establish a separate TS boundary among asubset of devices, and thus become a second leader.

Reference is now made to FIG. 12. As seen in FIG. 12, a first leader1210 establishes a first TS boundary among various devices, such asdevices 1212, 1214, 1216, 1218, 1220 and 1222.

A second device 1230 misses the TS boundary establishment from leader1210 and thus transmits a PS signal to establish a TS boundary. Devices1232 and 1234 also missed the original TS boundary signal from device1210 and therefore use the TS boundary as provided by device 1230.

In order to overcome having two TS boundaries within the cluster ornetwork, one option is to reset a network once the misalignment of theTS boundary is discovered. This can be done by designating a RESETsignal on the PS channel so that a device can notify other devices whenit experiences TS boundary misalignment. In one embodiment, the RESETsignal could be a PS signal with a special SSS sequence, for example.

In the example of FIG. 12, device 1222 detects signals from both leader1210 and leader 1230 and realizes that there is a TS boundarymisalignment between the two. Thereafter, device 1222 transmits a resetsignal and further resets itself.

Similarly, device 1234 can detect the TS boundary misalignment when itreceives PS signals for devices under leader 1210 and it may alsotransmit a reset signal on its PS channel for the duration of one cycle.

In some cases, the reset may be performed on a particular network. Forexample, a smaller network may be reset rather than a larger network inorder to avoid convergence issues with larger networks. In other cases,groups or networks that have higher priority data payloads may requestthe other network with the TS boundary misalignment reset itselfinstead. In other cases both networks may reset themselves to start theentire process over. Once a reset is received, the process of FIG. 14may be restarted and the process of FIG. 15 may be used to re-establisha new TS boundary.

The second issue caused by the random link failures described aboverelates to a stochastic hidden node problem. Two scenarios for suchhidden node problems exist. A first occurs when two devices pick thesame TS to transit their PS at the very first attempt. The second occurswhen one device is already transmitting its PS on a time-slot, but dueto the link failure a new device may not detect the existing PS and maypick the same TS to transmit its own PS.

The consequence of both scenarios is that a first device and a seconddevice collide but the collision may never be resolved due to randomlink failures in the network. Specifically, as shown with regard to FIG.13, where a first device 1310 and a second device 1312 both transmit,there are some devices such as device 1316 and device 1318 that onlyreceive signals from one of the transmitters. In other words, device1316 may receive a signal only from device 1310 and device 1318 mayreceive a signal only from device 1312 in the example of FIG. 13.

Although other devices in the network experience the collision betweenthe signals of device 1310 and device 1312 and do not send an ACK,device 1316 will send an ACK to device 1310 since it only received thesignal from device 1310. Similarly, device 1318 will send an ACK todevice 1312 since it received a signal only from device 1312.

Devices 1310 and 1312 will receive ACK signals and believe that theyhave secured a TS. The devices 1310 and 1312 will never know about thecollision and will therefore never jump to a new TS to avoid thecollision.

In order to overcome the above, one solution is based on the observationthat wireless links undergo independent quasi-static fading, whereby anyreceiving device will experience a mixture of the correct PS signals,corrupted PS signals and missed signals over a number of cycles. Theenhanced capability of detecting the collision of PS signals is enabledby the use of SSS signals as a secondary presence signal. When two ormore SSS signals combine, such signals can be detected and a count ofthe presence of multiple devices on the same time-slot be made. Based onthis, one embodiment of the present disclosure provides for a “TSConfidence Level” in order to determine whether the UE should stay inits TS or move to another TS.

In one embodiment, a TS Confidence Level may be designed for time-slotson both the transmitter and receiver sides. At the receiver side, inaccordance with one embodiment, the receive TS confidence level of a TSis increased by one unit when a single PS signal is received in the TS.The receive TS confidence level of a TS decreases by one unit when morethan one signal is received or more than one SSS signals are detected inthat TS. If the receive TS confidence level of a TS is larger than apre-defined receive TS confidence threshold, an ACK signal will be sentout upon receiving a correct PS signal in that TS.

Similarly, at the transmitter side, the transmit TS confidence level ofa TS increases by one unit if an ACK is received for the PS transmittedin that TS. Similarly, the transmit TS confidence level of a TSdecreases by one unit if an ACK is not received for the PS transmittedon that TS. When the transmit confidence level is below a pre-definedthreshold, the transmitter UE relinquishes its secured TS before movingto search for a new TS.

Over a window of several discovery periods, if the number of times a PSsignal received in a given TS is significantly less than the number oftimes a PS signal is missing, the receiving devices can implicitly inferthat collisions must have occurred on that TS. For example, if a linkfailure rate is 10% and two devices collide, the receiving device willdetect combined PS signals of both colliding devices for 81% PS signalfrom either one of the colliding devices for 18% and no PS signals for1% of the time. As such, in this stochastic hidden node scenario, allreceiver nodes will eventually detect a collision with a higherprobability as their receive confidence level decreases with time.Therefore, such devices will stop sending ACK signals back to collidingtransmitters. After several counting periods, the colliding transmitterswill not receive enough ACK signals, resulting in a low transmit TSconfidence for their secured time-slot. Eventually such devices willmove to select a new random TS in which to transmit.

When a device only experiences radio link failures but no collisions,then the number of PS signals received is significantly larger than thenumber of missed ones. For example, if the link failure is 10% and thereceiving device will detect a PS signal for 90% of the time and no PSsignals for 10% of the time. Therefore, the receive TS confidence levelremains large enough for any receiver device to send an ACK signal backto the transmitter, even when there are some missing PS signals. Assuch, the present embodiment is robust to link failures.

The above may be implemented, for example, with reference to the processof FIG. 14. As seen in FIG. 14, the process starts or resets at block1410 and proceeds to block 1412 in which a check is made to determinewhether a reset has been received. If yes, then the process proceedsback to block 1410 in which a reset occurs and then back to block 1412.

If a reset has not been received the process proceeds to block 1420 todetermine whether or not a TS boundary has been established. If no, theprocess proceeds to a boundary establishment block 1430. One example ofa boundary establishment block is found in regard to FIG. 15.

In particular, referring to FIG. 15, the process enters at block 1510and proceeds to block 1512 in which a random contention time τ ispicked. The process then proceeds to block 1514 in which the devicelistens until a time D+τ.

From block 1514 the process proceeds to block 1516 in which a check ismade to determine whether a PS was received during the probing time D+τ.As will be appreciated by those in the art, the check at block 1516 isan ongoing check and could occur at any time during the D+τ timeinterval and does not need to wait until the end of the D+τ timeinterval.

From block 1516, if a PS is received then the process proceeds to block1520 in which a TS boundary is established. The process then proceeds toblock 1522 in which the device listens for one discovery cycle to pick afree TS and the process then proceeds to block 1524 in which a check ismade to determine whether any free TS has been found. If no free TS wasfound in block 1524 the process proceeds to block 1530 and ends sincethere are no free TSs available for the device to communicate.

Conversely, if a free TS is found then the device proceeds to block 1532in which a free TS is randomly chosen and the process proceeds to block1560 in which the process leaves the boundary establishment block.

From block 1516 if a PS is not received during the probing time intervalthen the process proceeds to block 1540 in which the device itselftransmits a PS in order to initiate a TS boundary.

The process then proceeds to block 1542 in which a check is made todetermine whether the acknowledgement time has yet arrived. If no thenthe process proceeds to an idle block 1544 and then proceeds back toblock 1542.

Once the acknowledgement time has arrived the process proceeds to block1550 and checks whether or not an ACK is received. If yes, the processproceeds to block 1552 in which a TS boundary is established.

If from block 1550, the now ACK is not received, or after block 1552,the process proceeds to block 1560 in which the TS boundaryestablishment block is exited.

As will be appreciated by those in the art, if an ACK is not received atblock 1550 then there is no TS boundary establishment at that time andthe process would proceed back to block 1430 in FIG. 14.

Referring back to FIG. 14, once the boundary establishment at block 1430has been exited, the process proceeds to block 1460 in which the deviceproceeds to the next TS. From block 1460 the process proceeds back toblock 1412 to check if a reset has been received or not.

During the check at block 1420, if the TS boundary has been establishedthen the process proceeds to block 1440 in which a check is made todetermine whether the current TS is either a transmit PS time-slot or areceive ACK time-slot (i.e. a discovery time-slot for the device). Ifyes then the process proceeds to block 1442 which is a PS transmissionblock. One example of a PS transmission block is shown with regard toFIG. 16.

In particular, referring to FIG. 16, the process begins at block 1610and proceeds to block 1612 in which a check is made to determine whetherthe TS is used for transmitting a PS. If yes, the process then proceedsto block 1620 in which the PS is transmitted and the process thenproceeds to block 1630 in which the PS transmission block is exited.

If the TS is not for transmitting the PS as determined at block 1612,the process proceeds to block 1640 in which a check is made to determinewhether an ACK has been received at the device. If yes, the processproceeds to block 1642 and the transmit TS confidence is increased byone unit. Conversely, if no ACK has been received the process proceedsto block 1644 in which the transmit TS confidence is decreased by oneunit.

The process then proceeds from block 1642 or block 1644 to block 1650 inwhich a check is made to determine whether the transmit TS confidence isless than a pre-defined threshold. If the transmit TS confidence is notless than the threshold then the device is confident regarding the TSselection and the process proceeds back to block 1630 and exits the PStransmission block.

Conversely, if the transmit TS confidence is below the threshold thenthe process proceeds from block 1650 to block 1652 where the devicelistens for one discovery cycle to pick a new free TS.

The process then proceeds to block 1660 in which a check is made todetermine whether there are any free TSs. If not, the device proceeds toblock 1662 and ends the process as there are no free TSs available forcommunication.

Conversely, if there are free TSs then the process proceeds from block1660 to block 1664 in which a new TS is randomly picked from theavailable free TSs. From block 1664 the process proceeds to block 1630and exits the PS transmission block.

Referring again to FIG. 14, once the process exits the PS transmissionblock the process then proceeds to block 1460 in which the device goesto the next TS and then back to block 1412 to check whether a reset hasbeen received.

From block 1440, if the TS is not the one used for transmitting the PSor receiving the ACK then the process proceeds to block 1450, which isan ACK transmission block. Reference is now made to FIG. 17, which showsone example of an ACK transmission block.

The ACK transmission block is entered at block 1710 and the processproceeds to block 1712 in which a check is made to determine whether aPS has been received in the TS. If yes, the process proceeds to block1720 in which a check is made to determine whether the TS is misaligned.The misalignment of the TS may be detected for example, by receivingmultiple PSs or PSs that have TS boundaries that are different than theTS established for the device.

The process proceeds from block 1720 to block 1722 if there is amisalignment. At block 1722, a RESET is transmitted by the device andthe process then proceeds to block 1724 in which the device itselfresets its TS boundaries.

From block 1724 the process proceeds to block 1730 and exits the ACKtransmission block.

If the TS is not misaligned, as determined at the check of block 1720,the process proceeds to block 1740 in which a check is made to determinewhether a collision has been detected in the TS. If a collision has beendetected then the process proceeds to block 1742 in which the receive TSconfidence is decreased by one unit and the process then proceeds toblock 1730 and exits the ACK transmission block.

If a collision is not detected then the process proceeds to block 1744in which the receive TS confidence is increased by one unit and theprocess then proceeds to block 1750 in which a check is made todetermine whether the receive TS confidence is greater than apre-defined threshold. If the receive TS confidence is not greater thanthe threshold the process proceeds to block 1730 and exits the ACKtransmission block.

If the receive TS confidence is greater than the threshold as determinedby block 1750 then the process proceeds to block 1752 in which thedevice transmits an ACK and sets its TS status to 1.

From block 1752 the process proceeds to block 1730 and exits the ACKtransmission block.

If a PS is not received in the time-slot, the process proceeds fromblock 1712 to block 1760 in which a check is made to determine whetherthe TS status is 1. If no, the process proceeds to block 1730 and exitsthe ACK transmission block.

If the TS status is 1, the process proceeds from block 1760 to 1762 inwhich the receive TS confidence is decreased by one unit.

The process then proceeds to block 1764 in which a check is made todetermine whether the receive TS confidence is less than a secondpre-defined threshold. If no the process proceeds to block 1730 andexits the ACK transmission block.

However, if the receive TS confidence is less than the second thresholdthe process proceeds to block 1766 and sets the TS status to 0. Fromblock 1766 the process proceeds to block 1730 and exits the ACKtransmission block.

From the above, the setting of the time-slot status of FIG. 17 to oneindicates confidence in the TS.

Referring again to FIG. 14, once the ACK transmission block at block1450 is exited then the process proceeds to block 1460 in which thedevice proceeds back to the next TS.

Reference is now made to FIG. 18, which shows a plot of the time toreach convergence with N_(Max) set at 32 and the number of devicesactually attempting to reach convergence varying from 2 to 32. As seenin FIG. 18, various link failure rates are provided and the number ofcycles required for network convergence is shown. As can be seen, up to10% link failure rates, the number of cycles for convergence linearlyincreases with the number of devices in the network. For 20% linkfailure rates, the linearity is broken down when the number of devicesis more than 20. This is because high link failure rate creates a largenumber of stochastic hidden nodes and not enough time-slots for thedevices to select from.

ID Selection

In a further embodiment, when convergence is achieved, all N_(Max)number of time-slots or some of them may be occupied. Each devicereserves one time-slot. Therefore, if there are enough devices in thelocal neighborhood, then the converged device state M=N_(Max) number ofdevices are able to join the network. If there are not enough devices,such that M<N_(Max), some time-slots remain empty. As each device isable to decode M−1 presence signals per discovery period, each devicecan count that there are M established devices. If the number ofestablished devices does not change for two consecutive cycles of thediscovery period, then all established devices assume that a convergedstate has been reached and an identifier selection process can beinitiated in accordance with one embodiment.

To this end, all established devices select unique IDs from a pool ofbits of binary codes. The pool may, for example, be [log₂(M)]. Thedevice transmitting on a first time-slot can select its identifierrandomly by picking any [log₂(M)] bits of binary code and broadcast iton the same OFDM symbol that also carries its presence signal. All otherM−1 devices receive the ID selected by the first device. The device thattransmits on the second time-slot excludes the ID used by the firstdevice and randomly picks another [log₂(M)] bits of binary code from theremaining pool. The third device excludes the IDs selected by the firsttwo devices from its pool and the process continues until the lastdevice selects and broadcasts its ID.

In this way each device receives a unique identifier to allow directedcommunications between devices.

Device Arrival And Departure

In a further embodiment, a device may leave a cluster. This can happenbecause the device is turned off or the device moves away from the peerdevices or the user of the device deactivates D2D mode and thus will nottransmit any D2D type signals.

When a device leaves, peer devices will detect that the device discoverysignal is declining. When it drops below the thresholds as definedabove, the peer devices determine that the device has left the cluster.If the PS signal on any time-slot is missing for, for example, threeconsecutive cycles, then the corresponding device is assumed to havedeparted. The number 3 is however only one example and other predefinednumbers may be established.

If the first device that can establish a TS boundary needs to depart,then another established device in the network can be randomly assignedto transmit the SFN. Correspondingly, the leaving device may sense theweakening signalling strength of other devices in the group. When thereference signal received power (RSRP) of the cluster's PS is below apredefined threshold, the leaving device should stop transmitting the PSand detach from the cluster.

Any new device can join a network as long as the free time-slot isavailable within the discovery period. Each newly arriving device mustfirst probe the channel for a full discovery cycle to learn the TSboundaries and available time-slots, and then transmit a PS on theavailable time-slot and wait for an ACK from other devices.

Adjusting the Periodicity of the Discovery Cycle

In a further embodiment, the periodicity of the discovery cycle may beadjusted. For example, when the network is in a state of convergence andthe number of devices established is either much less than the totalnumber of TSs per discovery period, or when the number of devicesapproaches the maximum number of devices in the maximum number oftime-slots in the discovery cycle, the periodicity may be adjusted.Similarly, whenever a new device arrives or established device leaves acluster then the periodicity might need to be adjusted.

The adjusting of the periodicity may be achieved by broadcastingadjustment, so that established devices collectively realign to the newdiscovery cycle, which could be larger or smaller.

To increase the probability that all devices become aware of the newperiodicity, various options are possible. In one, a device may have aleader or master that determines new periodicity and that master maybroadcast a message assigning the new periodicity at predefined TSs. Themessage may include a start time of the change.

A subset of the cluster members may echo the cluster master's message byrepeating it. The subset of members may be, for example, those that havethe lowest IDs. However, in some embodiments all members may repeat andin other embodiments the selection of the repeating cluster members maybe chosen based on other criteria.

At the announced start time, all devices in the cluster then adjust tothe new PS period.

The above may, for example, be implemented by changing the 3GPP TS36.211 “Evolved Universal Terrestrial Radio Access (E-UTRA); Physicalchannels and modulation”, v. 11.3.0, June 2013, the entire contents ofwhich are incorporated herein by reference. In particular, the boldsection in Table 1 below may be added.

TABLE 1 Amendments to TS 36.211 3GPP 36.211 4 Frame structure Downlinkand uplink transmissions are organized into radio frames with T_(f) =307200 × T_(s) = 10 ms duration.

 Three radio frame structures are supported: Type 1, applicable to FDD,Type 2, applicable to TDD. Type 3, applicable to device-to-devicediscovery and communication. 4.2 Frame structure type 3 Fordevice-to-device discovery and communication, frame structure type 3 isused. In frame structure type 3, a subframe is a downlink or uplinksubframe in reference to the transmitting device. A subframe is a uplinksubframe in reference to the transmitting device, and a downlinksubframe in reference the receiving device. At least one subframe forevery N _(max) subframes is a uplink subframe in reference to a giventransmitting device, over which presence signal of the concernedtransmitting device is transmitted. The parameter N _(max) isconfigurable. 5.9 Presence Signals For device-to-device discovery andcommunication, a UE transmits presence signal periodically. The presencesignal a UE transmits contains a sequence that the UE selects from apredefined set of sequences. The correct reception of a presence signalis acknowledged by a ACK/NACK sequence transmission.

The above may be implemented by any UEs. One exemplary device isdescribed below with regard to FIG. 19.

UE 1900 is typically a two-way wireless communication device havingvoice and data communication capabilities. UE 1900 may have thecapability to communicate with other UEs and in some instance tonetworks. Depending on the exact functionality provided, the UE may bereferred to as a data messaging device, a two-way pager, a wirelesse-mail device, a cellular telephone with data messaging capabilities, awireless Internet appliance, a wireless device, a mobile device, or adata communication device, as examples.

Where UE 1900 is enabled for two-way communication, it may incorporate acommunication subsystem 1911, including both a receiver 1912 and atransmitter 1914, as well as associated components such as one or moreantenna elements 1916 and 1918, local oscillators (LOs) 1913, and aprocessing module such as a digital signal processor (DSP) 1920. As willbe apparent to those skilled in the field of communications, theparticular design of the communication subsystem 1911 will be dependentupon the communication system in which the device is intended tooperate. The radio frequency front end of communication subsystem 1911can be used for any of the embodiments described above.

If enabled for network connection as well as D2D connection, UE 1900 mayhave network access requirements that will vary depending upon the typeof network. In some networks network access is associated with asubscriber or user of UE 1900. A UE may require a removable useridentity module (RUIM) or a subscriber identity module (SIM) card inorder to operate on a CDMA network. The SIM/RUIM interface 1944 isnormally similar to a card-slot into which a SIM/RUIM card can beinserted and ejected. The SIM/RUIM card can have memory and hold manykey configurations 1951, and other information 1953 such asidentification, and subscriber related information.

When required network registration or activation procedures, if any havebeen completed, UE 1900 may transmit and receive communication signalsover the network. Otherwise, network registration can occur inaccordance with the embodiments above for a D2D network.

Signals received by antenna 1916 are input to receiver 1912, which mayperform such common receiver functions as signal amplification,frequency down conversion, filtering, channel selection and the like.A/D conversion of a received signal allows more complex communicationfunctions such as demodulation and decoding to be performed in the DSP1920. In a similar manner, signals to be transmitted are processed,including modulation and encoding for example, by DSP 1920 and input totransmitter 1914 for digital to analogue conversion, frequency upconversion, filtering, amplification and transmission via antenna 1918.DSP 1920 not only processes communication signals, but also provides forreceiver and transmitter control. For example, the gains applied tocommunication signals in receiver 1912 and transmitter 1914 may beadaptively controlled through automatic gain control algorithmsimplemented in DSP 1920.

UE 1900 generally includes a processor 1938 which controls the overalloperation of the device. Communication functions, including data andvoice communications, are performed through communication subsystem1911. Processor 1938 also interacts with further device subsystems suchas the display 1922, flash memory 1924, random access memory (RAM) 1926,auxiliary input/output (I/O) subsystems 1928, serial port 1930, one ormore keyboards or keypads 1932, speaker 1934, microphone 1936, othercommunication subsystem 1940 such as a short-range communicationssubsystem and any other device subsystems generally designated as 1942.Serial port 1930 could include a USB port or other port known to thosein the art.

Some of the subsystems shown in FIG. 19 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 1932 and display1922, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the processor 1938 may be stored in apersistent store such as flash memory 1924, which may instead be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile memory such as RAM 1926. Received communication signals mayalso be stored in RAM 1926.

As shown, flash memory 1924 can be segregated into different areas forboth computer programs 1958 and program data storage 1950, 1952, 1954and 1956. These different storage types indicate that each program canallocate a portion of flash memory 1924 for their own data storagerequirements. Processor 1938, in addition to its operating systemfunctions, may enable execution of software applications on the UE. Apredetermined set of applications that control basic operations,including at least data and voice communication applications forexample, will normally be installed on UE 1900 during manufacturing.Other applications could be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storagemedium. The computer readable storage medium may be a tangible or intransitory/non-transitory medium such as optical (e.g., CD, DVD, etc.),magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM)application having the ability to organize and manage data itemsrelating to the user of the UE such as, but not limited to, e-mail,calendar events, voice mails, appointments, and task items. Naturally,one or more memory stores would be available on the UE to facilitatestorage of PIM data items. Such PIM application may have the ability totransmit and receive data items. Further applications may also be loadedonto the UE 1900, for example through an auxiliary I/O subsystem 1928,serial port 1930, short-range communications subsystem 1940 or any othersuitable subsystem 1942, and installed by a user in the RAM 1926 or anon-volatile store (not shown) for execution by the processor 1938. Suchflexibility in application installation increases the functionality ofthe device and may provide enhanced on-device functions,communication-related functions, or both.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem1911 and input to the processor 1938, which may further process thereceived signal for output to the display 1922, or alternatively to anauxiliary I/O device 1928.

A user of UE 1900 may also compose data items such as email messages forexample, using the keyboard 1932, which may be a complete alphanumerickeyboard or telephone-type keypad, among others, in conjunction with thedisplay 1922 and possibly an auxiliary I/O device 1928. Such composeditems may then be transmitted over a communication network through thecommunication subsystem 1911.

For voice communications, overall operation of UE 1900 is similar,except that received signals would typically be output to a speaker 1934and signals for transmission would be generated by a microphone 1936.Alternative voice or audio I/O subsystems, such as a voice messagerecording subsystem, may also be implemented on UE 1900. Although voiceor audio signal output is generally accomplished primarily through thespeaker 1934, display 1922 may also be used to provide an indication ofthe identity of a calling party, the duration of a voice call, or othervoice call related information for example.

Serial port 1930 in FIG. 19 would normally be implemented in a personaldigital assistant (PDA)-type UE for which synchronization with a user'sdesktop computer (not shown) may be desirable, but is an optional devicecomponent. Such a port 1930 would enable a user to set preferencesthrough an external device or software application and would extend thecapabilities of UE 1900 by providing for information or softwaredownloads to UE 1900 other than through a wireless communicationnetwork. The alternate download path may for example be used to load anencryption key onto the device through a direct and thus reliable andtrusted connection to thereby enable secure device communication. Aswill be appreciated by those skilled in the art, serial port 1930 canfurther be used to connect the UE to a computer to act as a modem.

Other communications subsystems 1940, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between UE 1900 and different systems ordevices, which need not necessarily be similar devices. For example, thesubsystem 1940 may include an infrared device and associated circuitsand components or a Bluetooth™ communication module to provide forcommunication with similarly enabled systems and devices. Subsystem 1940may further include non-cellular communications such as WiFi or WiMAX.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

1-26. (canceled)
 27. A method at a first device for enabling adevice-to-device wireless link, the method comprising: detecting whethera presence signal of a second device is received over a first timeperiod, the presence signal of the second device having a time-slotboundary; if the presence signal of the second device is not detected,initiating a time-slot boundary by the first device including: randomlyselecting a first presence signal based on Zadoff-Chu root sequenceindexes different from ones used by a base station; transmitting thefirst presence signal of the first device in a selected time-slot and aradio frame number starting from
 0. 28. The method of claim 27, whereinthe first device transmits the first presence signal in the selectedtime-slot during a subsequent frame, wherein a frame comprises aninteger number of time-slots.
 29. The method of claim 27, wherein atime-slot duration is predetermined.
 30. The method of claim 27, whereinthe first time period comprises a predetermined frame period and anon-deterministic contention period.
 31. The method of claim 30, whereinthe frame period comprises an integer number of time-slot durations. 32.The method of claim 27, further comprising: if the presence signal ofthe second device is detected, aligning a time-slot boundary on thefirst device to the time-slot boundary established by the second device;and after aligning to the time-slot boundary established by the seconddevice, determining a time-slot to use for transmission by the firstdevice.
 33. The method of claim 32, wherein the determining thetime-slot comprises: detecting one or more free time-slots; choosing afree time-slot; transmitting the first presence signal on the chosenfree time-slot.
 34. The method of claim 33, further comprising: checkingfor an acknowledgment to the first presence signal.
 35. The method ofclaim 27, further comprising sending an acknowledgment to a receivedpresence signal of any device other than the first device.
 36. Themethod of claim 27, wherein the first presence signal comprises one ormore sequences.
 37. The method of claim 36, wherein the first presencesignal comprises a primary synchronization signal and a secondarysynchronization signal.
 38. The method of claim 37, wherein the primarysynchronization signal is used to establish the time-slot boundary. 39.The method of claim 36, wherein one or more of the sequences variesbetween two consecutive presence signal transmissions.
 40. The method ofclaim 36, wherein one or more of the sequences stays the same for aplurality of consecutive presence signal transmissions.
 41. The methodof claim 27, wherein the selection of the first presence signal isrelated to an identity of the first device.
 42. A method at a firstdevice for enabling a device-to-device wireless link, the methodcomprising: listening for a presence signal on a channel, the presencesignal based on Zadoff-Chu root sequence indexes different from onesused by a base station and received in a selected time-slot and a radioframe number starting from 0; and upon detection of the presence signal,transmitting an acknowledgement to the presence signal; and aligning toa time-slot boundary associated with the presence signal by utilizingthe at least one sequence of the presence signal.
 43. The method ofclaim 42, wherein the presence signal comprises a Zadoff-Chu sequence.44. The method of claim 42, wherein the presence signal comprises aprimary synchronization signal and a secondary synchronization signal.45. The method of claim 44, wherein a sequence of the primarysynchronization signal relates to an identity of a plurality of devices.46. The method of claim 44, wherein the sequence of the secondarysynchronization signal relates to an identity of a device that transmitsthe presence signal.
 47. The method of claim 46, wherein the firstdevice uses the secondary synchronization signal to identify signalcollisions.
 48. A method at a device for enabling a device-to-devicewireless link, the method comprising: listening for a presence signalfrom another device on a channel; randomly selecting a first presencesignal based on Zadoff-Chu root sequence indexes different from the onesused by a base station; and transmitting the presence signal on thechannel to establish a time-slot boundary, wherein the transmitting ofthe presence signal in a selected time-slot and a radio frame numberstarting from
 0. 49. The method of claim 48, wherein the listening isdone for a first time period which comprises a predetermined frameperiod and a randomly generated contention period.
 50. The method ofclaim 48, wherein the frame period comprises an integer number oftime-slot durations.
 51. The method of claim 48, wherein the presencesignal comprises a primary synchronization signal and a secondarysynchronization signal.